Microstructured segmented electrode film for electronic displays

ABSTRACT

The display panel comprises surrounding media containing particles which are responsive to changes in a magnetic field are optically anisotropic toward the viewing surface. The electrode film has an array layer and an electrode layer, where the array layer provides a support structure for the electrode layer and electrically non conductive and in contact with the surface of the display panel and having. a plurality of mesa shaped segments where each mesa shaped segment has a top face and side walls extending downward from the top face. The electrode layer formed of an electrically conductive material coats the array layer and is exposed for contact by an electrical stimulus. The electrode layer is thicker on the top face than on the side walls, producing resistive bridges between adjacent top faces. The resistive bridges partially electrically isolate each electrode from the other electrodes in the electrode layer.

BACKGROUND OF THE INVENTION

The present invention relates to visual displays, and more particularlyto addressable, reusable, paper-like visual displays, such as “gyricon”(or twisting particle) displays or other forms of electronic paper, suchas particulate electrophoretic displays, which are available from E-inkCorporation. Specifically, the invention relates to substrates usable asa writing surface for gyricon displays or electronic paper.

A gyricon display, also called a twisting-ball display, rotary balldisplay, particle display, dipolar particle light valve, etc., offers atechnology for making a form of electric paper and other electronicallycontrolled displays. Briefly, a gyricon display is an addressabledisplay made up of a multiplicity of optically anisotropic particles,with each particle being selectively rotatable to present a desired faceto an observer. For example, a gyricon display can incorporate “balls”where each ball has two distinct hemispheres, one black and the otherwhite, with each hemisphere having a distinct electrical characteristic(e.g., zeta potential with respect to a dielectric fluid) so that theball is electrically as well as optically anisotropic. The balls areelectrically dipolar in the presence of the fluid and are subject torotation. A ball can be selectively rotated within its respectivefluid-filled cavity, for example, by application of an electric field,so as to present either its black or white hemisphere to an observerviewing the surface of the sheet.

A reflective image is formed by the pattern collectively created byindividual black and white hemispheres. By the application of anelectric field addressable in two dimensions (as by a matrix addressingscheme), the black and white sides of the balls are controlled as theimage elements (e.g., pixels or subpixels) of a displayed image.Alternatively, the display may be controlled by shaped electrodes toform one or more fixed images.

The balls are typically embedded in a sheet of optically transparentmaterial, such as an elastomer sheet. A dielectric fluid, such as adielectric plasticizer, is used to swell the elastomer sheet containingthe balls. Through this swelling, the dielectric fluid effectivelycreates a fluid-filled cavity around each ball. The fluid-filled cavityaccommodates the ball and allows the ball to rotate within itsrespective fluid-filled cavity, yet prevents the ball from migratingwithin the sheet.

When an electric field is applied to the sheet over a bead, theelectrical force on the bead overcomes the frictional adhesion of thebead to the cavity wall and causes the bead to rotate. Once rotation iscomplete, each bead will remain in a fixed rotational position withinits cavity. Thus, even after the electric field is removed, thestructures (balls) will stay fixed in position until they are dislodgedby another electric field. This bistability of the beads enables thegyricon display to maintain a fixed image without power. The bistabilityof a gyricon display is beneficial over other types of displays such asa liquid crystal display (LCD) or a light emitting diode (LED) displaywhich consume energy to maintain an image. Gyricon displays are thusparticularly useful for displays which will show an image for aprolonged period of time and only periodically have the image changed.

Gyricon displays are not limited to black and white images, as gyriconand other display mediums are known in the art to have incorporatedcolor. Gyricon displays have been developed incorporating eitherbichromal color, trichromol color, or four quadrant colored balls. Alsodeveloped are three or four segmented colored balls, as disclosed inU.S. Pat. No. 6,128,124 ( Silverman, ADDITIVE COLOR ELECTRIC PAPERWITHOUT REGISTRATION OR ALIGNMENT OF INDIVIDUAL ELEMENTS), incorporatedby reference herein.

The colored balls can be charged by adsorption of ions from a liquidonto the ball surface. Alternatively, colored balls can be charged byelectret formation by injection of an external charge into the surfaceregion of a colored ball, as is disclosed in U.S. Pat. No. 6,072,621(Kishi, COLOR BALL DISPLAY SYSTEM), incorporated by reference herein.

Like ordinary paper, electric paper preferably can be written on anderased, can be read in ambient light, and can retain imposed informationin the absence of an electric field or other external retaining force.Also like ordinary paper, electric paper preferably can be made in theform of a lightweight, flexible, durable sheet that can be folded orrolled into tubular form about any axis and can be conveniently placedinto a shirt or coat pocket and then later retrieved, restraightened,and read substantially without loss of information. Yet unlike ordinarypaper, electric paper preferably can be used to display full-motion andchanging images as well as text. While gyricon displays are particularlyuseful for displays where real-time imagery is not essential, gyricondisplays are adaptable for use in a computer system display screen or atelevision.

Gyricon display arrangements have typically taken one of three forms:(1) a slurry coat with balls randomly dispersed in a relatively thickfilm, (2) a monolayer where balls are closely packed in a layer; or (3)a dual layer, where balls are closely packed in a first layer and asecond layer of balls is provided to fill in the voids. To createdisplays which appear brighter with sharper images, gyricon displaysshould have high light reflectance. One way to improve the reflectanceof a monolayer gyricon display is to closely pack the bichromal balls.However, in dual or multiple layer displays, the packing density of theballs may be of little consequence insofar as overall displayreflectance is concerned, because balls located farther from the viewingsurface of the gyricon display will “fill in the gaps” between ballslocated nearer the viewing surface. So long as the two-dimensionalprojection of the balls onto the viewing surface at all distances fromthe viewing surface substantially covers the viewing surface, ahigh-quality display will be obtained.

In the context of gyricon displays, the “balls” are not necessarilyperfectly round or hemispherical. Instead of balls, a gyricon displaycan use substantially cylindrical bichromal particles rotatably disposedin a substrate. The twisting cylinder display has certain advantagesover the rotating ball gyricon display because the bichromal elementscan achieve a higher packing density. The higher packing density leadsto improvements in the brightness of the twisting cylinder display ascompared to the rotating ball gyricon display.

One drawback to twisting particle displays (using balls, cylinders,etc.) is that the quality of the image viewed is dependent on therotatability of the structures within the fluid. In practice, a particlemay not rotate completely or not at all, thus only partially exposingthe white or black color or a mix therebetween. Incomplete rotation ornon-rotation causes a loss in image contrast and color purity. It istherefore desirable to improve the resolution of the image on thedisplay by improving the rotatability of the structures within thefluid.

To achieve still higher packing density, a gyricon display can beconstructed without elastomer and without cavities. In such a display,the bichromal balls are placed directly in the dielectric fluid. Theballs and the dielectric fluid are then sandwiched between two retainingmembers (e.g., between the addressing electrodes) with no elastomersubstrate.

Substrates usable as a writing surface for Gyricon displays are known inthe prior art. EPO 942,405 A2 (Howard et al., “CHARGE RETENTION ISLANDSFOR ELECTRIC PAPER AND APPLICATIONS THEREOF”) discloses a pattern ofconductive charge retaining islands on the surface of a Gyricon sheet.

In addition to using the present invention with Gyricon displays, theinvention can also be used in combination with particulateelectrophoretic displays, such as available from E-Ink Corporation, orother electronic paper. A particulate electrophoretic display, such asavailable from E-Ink Corporation (or electronic ink) comprisestransparent “microcapsules” filled with a densely colored fluid such asa dark ink. Contained inside the micro capsule shell are hundreds oftiny solid spheres of a different color, such as brilliant whitetitanium dioxide, each of which has a negative charge. The microcapsules are typically sandwiched between a transparent conductive topelectrode and a bottom electrode. The negatively charged titaniumdioxide spheres are held against the bottom side of the micro capsule bya positive static electric field. When the particles are held againstthe bottom side, the white particles are submerged below the viewingsurface of the colored dye inside the micro capsules. When the polarityof the electric field is reversed, the micro capsules are repelled bythe negative field and are attracted to the transparent top electrodewhere the particles coat the top side viewing surface of the microcapsule. The coating of the viewing surface suddenly changes from thecolor of the dark ink to the color of the white spheres. Thus, aparticulate electrophoretic display, such as available from E-InkCorporation does not require the micro capsules to rotate in order toshow a change of color, but rather requires migration of the minuteparticles within the fluid contained in the micro capsule.

BRIEF SUMMARY OF THE INVENTION

The present invention is a micro structured film having a plurality ofisolated electrodes usable as a writing surface for a display panel forgyricon displays or electronic paper. The specific geometry of theisolated electrodes is an array of raised mesas having a semiconductivedeposition on the top layer. The individual mesa shaped electrodes areindividually addressable by a stylus or other electrical stimulus. Thedisplay panel has a viewing surface and a backside surface opposing theviewing surface. The display panel contains particles which areresponsive to changes in an electric or magnetic field and are opticallyanisotropic. The conductive electrode film is preferably transparent tovisible light.

The electrode film has an array layer and an electrode layer, where thearray layer provides a support structure for the electrode layer. Thearray layer is electrically non conductive and is disposed toward thedisplay panel. The array layer has a fabricated texture, such as aplurality of mesa shaped segments where each mesa shaped segmentcomprises a top face and side walls extending downward from the topface. The electrode layer is formed of an electrically conductivematerial and coats the array layer. The electrode layer is exposed forcontact by an electrical stimulus, such as a stylus. The electrode layercan be deposited by a sputtering process, wherein the electrode layer isthicker on the top face than on the side walls producing resistivebridges between adjacent top faces. The resistive bridges partiallyelectrically isolate each electrode from the other electrodes in theelectrode layer. Thus, each shaped electrode is capable of beingindividually addressed by a stylus without addressing other electrodesegments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a portion of a display in accordancewith the invention.

FIG. 2 is a sectional exploded view of the display of FIG. 1.

FIG. 3 illustrates an enlargement of a portion of the display of FIG. 1.

FIGS. 3A, 3B, 3C show a method in accordance with the invention ofmaking the display of FIG. 1.

FIG. 4 is a cross sectional view of the electrode film 12 of FIGS. 1 and2.

FIGS. 4A-E illustrate optional embodiments of the electrode film.

FIG. 5 is a photo of the surface of the microstructured inventiveelectrode film after having an electrical stimulus address the displayand write indicia thereon.

FIG. 6 is a sectional view of a display in accordance with the presentinvention where the display is viewed opposite I in FIG. 1.

FIG. 7 is a sectional view of a display in accordance with the presentinvention disposed in a printer utilizing the inventive electrode film.

FIG. 8 is a photo of the surface of the inventive electrode film showingthe size of the mesa shaped segments and the relative distance betweeneach mesa shaped electrode in the array of mesa shaped electrodes.

While the above-identified drawing figures set forth preferredembodiments of the invention, other embodiments are also contemplated,as noted in the discussion. In all cases, this disclosure presents thepresent invention by way of representation and not limitation. It shouldbe understood that numerous other modifications and embodiments can bedevised by those skilled in the art which fall within the scope andspirit of the principles of this invention. It should be specificallynoted that FIGS. 1 and 2 have not been drawn to scale, as it has beennecessary to enlarge certain portions for clarity. It should also benoted that FIGS. 1 and 2 show idealized representations of balls. In theapplication some balls may be irregularly shaped or have swirled ormixed colors and sometimes speckles on the balls.

DETAILED DESCRIPTION

FIG. 1 shows a visual display 10 having the inventive electrode film 12.The display 10 includes a bottom substrate 14 with a backside electrode16. A display panel 18 is sandwiched between the electrode film 12 andthe bottom substrate 14. The display panel 18 has bichromal particles 20disposed in a surrounding media 22 (not shown in FIG. 1 for clarity). Asshown in FIG. 1, an electrical potential source 23 is electricallygrounded to the backside electrode 16.

The electrode film 12 is preferably positioned on a viewing side of thedisplay and is preferably optically transparent. With an opticallytransparent electrode film, ambient light can impinge upon the displaypanel 18 through the electrode film 12 (from above in FIGS. 1 and 2),and ambient light incident upon the display panel 18 will reflect toprovide an image at I, as shown in FIG. 1.

The bottom substrate 14 is a nonconductive layer. The bottom substrate14 can be made of any electrically insulative material and is preferablya sheet of plastic. If flexibility of the display 10 is desired, thebottom substrate 14 should be flexible. The primary purpose of thebottom substrate 14 is to support the backside electrode 14 and toelectrically insulate the backside electrode against contact.

The backside electrode 16 is preferably positioned on the bottomsubstrate 14 toward the display panel 18. The backside electrode 16covers the surface of the bottom substrate 14 and can be made of anyelectrically conductive or semiconductive material. Alternatively, thebackside electrode 16 could cover the bottom surface of the bottomsubstrate 14. While the backside electrode 16 is a simple plateelectrode in the preferred embodiment shown, the backside electrode 16could alternatively be electronically addressable.

The particles 20 of the display panel 18 are preferably rotatingspheres, balls or beads. Alternatively, the particles can be of anyshape including twisting cylinders as disclosed in U.S. Pat. No.6,055,091 (Sheridon, “TWISTING-CYLINDER DISPLAY”), which is incorporatedby reference herein. The balls 20 are of the same type that is typicallyfound in Gyricon displays. Gyricon display technology is describedfurther in U.S. Pat. No. 4,126,854 (Sheridon, “TWISTING BALL PANELDISPLAY”) and U.S. Pat. No. 5,389,945 (Sheridon, “WRITING SYSTEMINCLUDING PAPER-LIKE DIGITALLY ADDRESSED MEDIA AND ADDRESSING DEVICETHEREFOR”), which are incorporated by reference herein.

The particles 20 are optically anisotropic, such as hemisphericallybichromal. The optical anisotropy of the particles 20 may be caused by acoating having a difference in Zeta potential, thereby causing theparticles 20 to have a corresponding electrical anisotropy. An acquiredelectrical charge is shown symbolically in FIG. 3 where dark hemispheres28 are more positive than light hemispheres 30. With the electricalanisotropy, particles 20 are subject to rotation such by an electric ormagnetic field powered by the electrical potential source 23 (shown inFIG. 1).

Gyricon displays are not limited to bichromal structures. Gyriconsincorporating color have been described in U.S. Pat. No. 5,760,761“HIGHLIGHT COLOR TWISTING BALL DISPLAY”, U.S. Pat. No. 5,751,268“PSEUDO-FOUR COLOR TWISTING BALL DISPLAY”, U.S. patent application Ser.No. 08/572,820 “ADDITIVE COLOR TRANSMISSIVE TWISTING BALL DISPLAY”nowU.S. Pat. No. 5,892497, U.S. patent application Ser. No. 08/572,780“SUBTRACTIVE COLOR TWISTING BALL DISPLAY”now U.S. Pat. No. 5,767,826,and U.S. Pat. No. 5,737,115 titled “ADDITIVE COLOR TRISTATE LIGHT VALVETWISTING BALL DISPLAY”, which are incorporated by reference herein.

The size of the particles 20 can be selected based upon the distancefrom which the display 10 is intended to be read and the desiredresolution of the display 10. For example, the particles 20 can be 0.05to 0.5 millimeters in diameter.

The particles can be made of many materials as known in the Gyricon art.For example, the particles can be formed of crystalline polyethylenewax. FIG. 3 is an enlarged view, showing a dark coating applied to onehemisphere 28 of each of the spheres 20 to exhibit optical absorptioncharacteristics, as illustrated by their dark shading, and a secondcoating is applied to the other hemisphere 30 of each of the spheres 20to exhibit light reflectance characteristics, as illustrated by theabsence of dark shading. The difference between the lightreflectance-light absorption characteristics of hemispheres 28 and 30provides the desired optical anisotropy. Specifically, the spheres 30could be formed of black polyethylene containing a charge activationagent in one hemisphere with a light reflective material, for example,titanium oxide filled polyethylene in the other hemisphere. Alternately,the black polyethylene and the titanium oxide could be sputtered onhemispheres 30 a, 30 b to provide the spheres 30 with the desired lightreflective and light absorptive hemispheres. Alternately, theanisotropic spheres 30 could be coated with differently coloreddielectric coatings, with a charge activation agent added to one of thecoatings. Black coatings may be obtained by the simultaneous evaporationof magnesium fluoride and aluminum in a vacuum chamber, whereas whitecoatings may be obtained by the slow deposition of indium.

The balls 20 are disposed in a surrounding media 22 which may includesupport material such as a transparent elastomer 24 swelled withdielectric fluid 26 (See FIGS. 2 and 3). The surrounding media 22permits the particles 20 to have the desired rotational freedom underapplication of a switching field, while the surrounding media 22sufficiently contacts the particles 20 so bistability is maintained,i.e., the particles 20 do not rotate absent the application of aswitching field. The surrounding media 22 prevents the particles formhaving translational freedom. The display panel 18 has an opticallytransmissive viewing surface 34.

As an alternative to using elastomer 24, the surrounding media 22 can bea dielectric fluid with no elastomer. U.S. Pat. No. 5,754,332 (Crowley,MONOLAYER GYRICON DISPLAY) and U.S. Pat. No. 5,825,529 (Crowley, GYRICONDISPLAY WITH NO ELASTOMER SUBSTRATE), both incorporated by referenceherein, disclose gyricon or twisting-ball displays in which opticallyanisotropic particles are disposed directly in a working fluid, such asa dielectric liquid, without an elastomer substrate or othercavity-containing matrix.

As shown in FIGS. 2 and 3, each of the spheres 20 is located within acavity 32 of the transparent support material 24. Cavities 32 have adiameter slightly larger than the diameter of spheres 20 so that spheres20 have the necessary rotational freedom without translational freedom.An optically transparent dielectric liquid 26 fills the voids betweenspheres 20 and cavities 32.

FIGS. 3A, 3B and 3C exemplify the method of forming the preferreddisplay 18. The preferred panel 18 is formed by thoroughly mixing theoptically anisotropic particles 20 with an uncured (flowable), opticallytransparent material, for example, an uncured elastomer such as DowCorning SYLGARD 182. The optically transparent material 24 then cured,such as in the case of SYLGARD 182 by rapid heating to an elevatedtemperature of about 140° C. and maintaining the elastomer 24 at thattemperature for about 10 minutes and then cooled to room temperature. Inits initially cured state as shown in FIG. 3A, the elastomer 24restricts the spheres 20 from either rotational or translationalmovement. Following curing of the support material 24, the supportmaterial 24 is placed in a dielectric liquid plasticizer 26, as shown inFIG. 3B, for a period of time, typically overnight, with the plasticizer26 at room temperature. For example, the dielectric liquid plasticizer26 can be silicone oil, such as Dow Corning 10 Centistoke 200 oil whenthe elastomer is SYLGARD 182. Another satisfactory elastomer/plasticizercombination is Stauffer and Waker V-53 elastomer with the above siliconeoil.

When the cured support material 24 is placed in the plasticizer 26, theplasticizer 26 is absorbed by the support material 24 resulting in aswelling of the support material 22. The spheres 20 are made of amaterial which does not readily absorb the plasticizer 26 at operatingtemperatures, with the result that the swelling of the support material24 creates voids (spherical cavities 32) around the spheres 20, as shownin FIG. 3C. The voids or cavities 32 are filled with the plasticizer 26and this structure allows easy rotation of the spheres 20, whilepermitting essentially no translation of spheres 20.

The support material 24 need not be an elastomer and in lieu thereof canbe a rigid plastic such as polyethylene, polystyrene or plexiglass.Encapsulation can be achieved with the encapsulant molten or dissolvedin a volatile solvent. An uncured rigid material such as an epoxy can beused as the encapsulant provided that it is light transparent. It isnecessary that the material of support material 24 absorb theplasticizer 26 in order that the cavities 32 may be formed. When thesupport material 24 is an elastomer, the spheres 20 can be plastics suchas polyethylene or polystyrene which do not absorb the plasticizer 26 atworking temperatures. When the support material 24 is plastic, thespheres 20 may be of a material, such as glass.

In an optional embodiment the display panel 18 is a particulateelectrophoretic display, such as available from E-Ink Corporation orelectronic paper display utilizing non-rotating microcapsules instead ofthe rotating gyricon particles. U.S. Pat. No. 5,930,026 to Jacobson etal. titled NONMISSIVE DISPLAYS AND PIEZOELECTRIC POWER SUPPLIESTHEREFOR, U.S. Pat. No. 6,120,588 to Jacobson titled ELECTRONICALLYADDRESSABLE MICROENCAPSULATED INK AND DISPLAY THEREOF, and U.S. Pat. No.6,130,774 to Albert et al. titled SHUTTER MODE MICROENCAPSULATEDELECTROPHORETIC DISPLAY each disclose a type of encapsulatedelectrophoretic display and are incorporated herein by reference.

The electrode film 12 of the present invention is positioned in contactwith the viewing surface 34 of the display panel 18. As shown in FIG. 4,the electrode film 12 preferably includes a support layer 36, an arraylayer 38 and a top electrode layer 40.

The support layer 36 and the array layer 38 can be formed together toprovide a support structure for the top electrode layer 40. Both thesupport layer 36 and the array layer 38 are electrically nonconductive.The support layer 36 and the array layer 38 are preferably transparent.

The array layer 38 provides a textured surface 42. As can be seen inFIGS. 1 and 2, the textured surface 42 includes a plurality of mesastructures 44 each having an electrode support face 46 extendinggenerally in a common plane. Each mesa structure 44 has peripheral sidewalls 48 around the electrode support face 46. The peripheral side wallsdefine valleys 50 relative to the electrode support faces 46. As bestshown in FIG. 4, the preferred electrode support faces 46 aresubstantially flat, and the preferred peripheral side walls 48 defineV-shaped valleys 50. The side walls 48 preferably recess at includedangles of 70° or greater relative to the top face 46.

For ease of manufacturing, the mesa shaped segments 44 are provided inan array having a repeatable mesa size and a repeatable distance betweeneach mesa shaped segment 44. As best shown in FIGS. 1 and 2, thepreferred electrode support faces 46 are square, each providing fourperipheral side walls 48 of equal length. Variations to the shape of thetop face and the number of side walls are also possible and within thescope of this invention. The shape and slope of the sidewalls can alsovary.

The mesas 44 are sized in accordance with the size of the particles 20and in accordance with the desire resolution of the display 10. Forinstance, the mesas 44 can measure anywhere from 1 to 50 microns inheight and 5 to 200 microns in length. The preferred mesas 44 measure100 microns square, with electrode faces 46 elevated 25 microns abovethe bottoms of the valleys 50. The valleys 50 measure 25 microns wideand extend in a crisscross pattern.

By forming the mesa structures 44 out of a film having layers of twodifferent materials, the height of the mesas 44 is easily repeatable inmanufacturing, without overly reducing the strength of the film. In thepreferred embodiment, the support layer 36 is a polyester (polyethyleneterephthalate or “PET”) backing with the array layer 38 being acopolyester heat seal layer over the support layer 36. The support layer36 combined with the array layer 38 can be formed as a PET/co-PET film.The co-PET polymer, also called “80-20”, is poly(ethyleneterephthalate-co-ethylene isophthalate), of which 80% by mole is theformer polymer type. The isophthalate component of the co-PET filmprovides a melt temp and a degree of crystallization below that of thePET layer. The physical effect of the isophthalate produces a co-PETlayer embossable below temperatures where the PET softens or melts. Thusthe overall film, which is biaxially oriented, is not relaxed on the PETside during thermal embossing of the co-PET side. The support (PET)layer 36 is preferably thicker than the array (co-PET) layer 38, such asa support layer thickness of approximately 1.2 mils and an array layerthickness of approximately 0.8 mils. As an alternative to the dualstructure of the support layer 36 and the array layer 38, the mesas 44could be formed into a single homogeneous layer of material.

The plurality of mesa shaped segments 40 are formed into the co-PETlayer. One method of forming the microstructured mesas 40 into theco-PET layer is to compression mold such as with a silicone rubber mold(not shown). The pattern used for the preferred structure was 200 LPI(lines per inch). Press conditions were 160 degrees C., and the time 3minutes, and the pressure 6 tons for a sheet measuring 6×8 inches. Thepress was cooled to 100 C. before removal of the sample.

The electrode layer 40 is deposited on top of the array layer 38 in arelatively thin coating. For example, in the preferred embodiment, theelectrode layer 40 is formed to be about 4 nanometers thick. Theelectrode layer 40 is formed of an electrically conductive material,exposed for contact. The top electrode layer 40, thus provides aplurality of electrodes 52. Each mesa shaped segment 44 of the arraylayer 38 provides a support structure for one of the electrodes 52. Thetop electrode layer 40 has an electrode thickness over each mesa shapedsegments 44 of sufficient thickness and sufficient conductivity to forma substantially conductive electrode plate 52.

In the preferred embodiment, the conductive layer 40 is formed of IndiumTin Oxide (ITO). ITO is a transparent conductor, allowing the displaypanel 12 to be viewed through the segmented electrode film 12.

The preferred method of forming the electrode layer 40 is throughdeposition such as by a sputtering process. It is believed that such adeposition process produces and electrode layer 40 which issignificantly thinner on the sidewalls between the mesas 44 than it isover the electrode support surface 46. Sputter coating offers manyadvantages over conventional polymer coating techniques. ITO is a veryexpensive material, and is also potentially a limited natural resource.Sputter coating is advantageous because sputter coating the ITO utilizesthe ITO very efficiently, depositing the ITO in a thinner layer thanother conventional coating processes (4 nanometers compared to 70nanometers). Secondly, a thinner deposition of ITO from sputter coatingresults in a higher light transmittance and clarity. The microstructuredsurface also exhibits reduced glare when compared to conventional ITOcoated polyester. Thirdly, it is an additive process, so no ITO iswasted by a removal process.

The electrode layer 40 has a electrode thickness over the mesa shapedsegments 44 of sufficient thickness and sufficient conductivity to formsubstantially conductive electrode plates 52. As a consequence of thesteep sidewall 48, the sputter coating process deposits a much thinnerlayer of ITO on the sidewalls 48 than it does on the top faces 46 of themesas 44. Thus, the electrode layer 40 has a valley thickness over theside walls 48 which is thinner than the top face thickness. The thinnerITO (valley thickness) has a much higher resistance than the thicker ITO(top face thickness). As a result, the valley thickness forms resistivebridges 51 between adjacent electrode plates 52. The resistive bridges51 partially electrically isolate each electrode 52 from the otherelectrodes 52 in the electrode layer 40. This enables one electrode 52in the array 40 to be selectively addressed (energized) withoutsubstantially addressing the surrounding electrodes 52.

In the preferred embodiment, sputter coating 40 of the ITO was performedin a roll to roll process under the following conditions: ITO (90:10),In2O3:SnO2, DC power 1.5 kW, Argon 200 sccm, Oxygen 3.6 sccm, pressure4.5 mTorr. At 32 per feet per minute web line speed, it is estimatedthat approximately 4.4 nanometers of ITO was deposited over the top face46 of the microreplicated surface. Additional web speeds of 16, 8, 4,and 2 feet per minute result in nominally 8.8, 17.6, 35 and 70 nanometerthick ITO coatings 40. The ITO thickness estimates are for the flat tops46 of the micro structured mesas.

Workers skilled in the art would understand that deposition of aconductive material, such as ITO, is not limited to sputter coating.Other techniques for depositing conductive material, including boilingthe metal in a vapor deposition process and electroless metaldeposition, are also possible. In addition, vapor coating the topsurface first would result in a smaller amount of conductive materialdeposited in the valleys. Alternatively, the mesas can be formedsubsequent to the deposition of a conductive coating in a one stepprocess, such as by stamping or by pressing the mesa shape into the topsurface. This one step process would decrease the amount of conductivecoating wasted by not involving a chemical etching process or a costlylaser ablation process.

Other methods are possible as an alternative to deposition coating of aconductor to provide the electrode layer 40. For instance, a hotmeltadhesive may be pressed onto a surface of a transferable conductor, suchas graphite. The mesa structures may be pressed into theadhesive/graphite so the graphite adheres to cover the mesa tops 46 in asubstantially conductive layer, while leaving the valleys 50substantially less conductive. If necessary to permit sufficientconductivity in the resistive bridges 51 between mesa tops 46, a thincoating of a conductor or semiconductor in the valleys 50 may beextended over the mesas in a uniform thickness prior to enhancing theconductivity of the mesa tops with the transferable conductor.

As an alternative to forming the mesa structures 44 prior to applyingthe electrode layer 40, the mesa pattern can be embossed subsequent tocoating the top surface with a conductive coating. Such a method wouldstretch the conductive coating in newly formed valleys, rendering themesa electrode tops substantially but not totally electrically isolatedfrom its neighbors.

Subsequent to deposition of a conductive coating, the electrode layer 40could also be textured. Texturing of the electrode layer 40 produces amatte or rough finish to the conductive coating 40. A textured surfacefinish tends to reduce glare on the display 10. The sidewalls 48 alsoreduce glare by preventing reflection from occurring in the valleys 50between the electrode plates 52.

Several different mechanisms can be used so the segmented mesa electrodelayer 12 is useful in writing on a gyricon display 18. As shown in FIG.1, a stylus 54 is connected to the electrical potential source 53. Thesource 53 provides an electrical differential of 100 Volts relative tothe backside electrode 16 place a charge on electrodes 52 which arecontacted by the stylus 54. Once an electrode 52 is activated, theparticles 20 contained in the display 18 rotate responsively to show adifference in color. In practice, a stylus 54 or other electricalstimulus can be used to selectively address one electrode 52 in thearray 40 or a series of electrodes 52 in the array 40 without addressingthe entire display 18. Thus, the stylus 54 acts as a writing instrumentto address only those electrodes 52 the stylus 54 contacts. Byaddressing the particles 20 of the corresponding contacted electrodesegments 50, the stylus 54 effectively writes indicia thereby producinga visible image.

As an alternative to the stylus, the display 10 could be addressed by aprinthead (not shown) having an array of contacts. The printhead couldtransfer charge quickly and simultaneously to several selectedelectrodes 52.

In an optional embodiment, the mesas could comprise an elastomericmaterial for better durability during use. Elastic mesas with aconductive coating would better withstand the forces applied to the topsurface from a stylus.

FIGS. 4a-4 d show alternative geometries for mesas. In one embodiment,as shown in FIG. 4a, the side walls 48 meet at a trough 49 forming atrapezoidal valley 55. In this embodiment, the sputter coating processdeposits a layer of ITO into the trough 49 creating a trough thickness.Because the trough 49 and the top face 46 are substantially parallel,the trough thickness is substantially equal to the top face thickness.Nonetheless, the thinner deposition over the sidewalls still results inresistive bridges 51 between electrodes.

In other optional embodiments, shown in FIGS. 4b through 4 d, the sidewall angle, side wall slope, valley width, valley depth, mesa height andmesa shape have been adjusted to tune the degree of isolation of eachelectrode by varying the thickness of the conductive coating on the topsurface. The embodiments of FIGS. 4b and 4 c have mesas with rounded topsurfaces 56, where FIG. 4b has curved valleys 57 and FIG. 4c hastrapezoidal valleys 55 and troughs 49. As a result, FIGS. 4b, 4 c canhave optical properties as a lens structure in addition to the segmentedelectrode properties. The teachings of M507-12.0016 regarding opticalenhancement structures are incorporated by reference. While convexstructures are shown in FIGS. 4b and 4 c, convex mesa tops could also beused.

A common problem associated with electric paper is that such devices aresubject to inadvertent tribo-electric writing. Electric charges appliedinadvertently by tribo-electric exchanges during handling can createelectric fields causing the image on the display to change. Thetribo-electric fields threaten image retention and stability for anydisplay using field addressed electric paper sheets.

To combat the effects of tribo-electric fields, the top electrode layercould be coated with a material that protects against inadvertent tribocharging on the outer surface. In addition, the valleys of the mesastructures could also be filled with a protective material 58. Anembodiment with filled valleys is shown in FIG. 4d. The material 58 inthe valleys also helps protect the electrode layer from frictional wearassociated with dragging the stylus across the electrode tops. Theprotective material is added after the conductive layer is added.Optionally, the material used for protecting against inadvertenttribo-charging could be partially conductive in the lateral directionwith respect to the display image.

In all embodiments, the electrodes are not entirely electricallyisolated from one another, but rather are connected with resistivebridges 51. Electrodes separated by resistive bridges 51 canalternatively be formed through the reverse mesa structure shown in FIG.4e. In FIG. 4e, the thermoplastic material is microfabricated with afemale pattern of reverse mesas corresponding to the male pattern shownin FIG. 4a. Instead of deposition coating with a conductive material,the reverse mesas are flooded with conductive or semiconductivematerial. The electrodes 52 over the reverse mesas are thussubstantially thicker than the resistive bridges 51 between electrodes52. The embodiment of FIG. 4e provides a smooth top surface, minimizingtribo-charging and wear similar to the embodiment of FIG. 4d.

Throughout these embodiments, the electrodes 52 described have ofsubstantially equal x and y dimensions. In all these variousembodiments, any of the described mesa shapes can extend for asubstantial distance in one direction (i.e., into the page). That is,the electrode film could have resistance bridge grooves betweensubstantially parallel conductors. Instead of one electrode film withmesa patterned formations, two electrode films could sandwiched thedisplay 18. The grooves of one electrode film are oriented orthogonallyto the grooves of the other electrode film. The two electrode filmsenable addressing and indexing selected crossover points to form animage.

FIG. 5 shows a photo of the preferred embodiment in which a stylus haswritten “MESA FILM” across the surface of the microstructured electrodefilm 12. Because each mesa electrode 52 is partially isolated from theother mesa electrodes 52 in the array 40, writing speed affects theresulting line width. Rapid strokes make narrow lines and slower strokesmake broader lines. When making rapid strokes, the image lagged thestroke by a noticeable fraction of a second. Line thickness “bleed over”could be improved by using a more optimized microstructure and using amore columnated sputtering process.

As an alternative to a transparent segmented electrode film, theembodiment shown in FIG. 6 has a bottom substrate 14 and a backsideelectrode 16 which are optically transparent. The display 10 viewed fromI (opposite I in FIG. 1) and through the bottom substrate 14 andbackside electrode 16. For instance, the backside electrode 16 could beformed of Indium Tin Oxide (ITO) which is a clear metallic conductor.The electrode layer 40 could then be formed of a metal, such asaluminum, thereby increasing the durability of the electrode film. Theprimary benefit of viewing the display 10 through an opticallytransparent electrode film 12 of the preferred embodiment is thatindicia can be written on the front side similar to paper, rather thanwriting a mirror image of the desired indicia on the non viewing side ofthe display.

The present invention offers substantial benefits over the structuredisclosed in EPO 942,405. The charge retaining islands disclosed in EPO942,405 A2 have narrow channels of insulating material to preventmigration of charge laterally across the sheet. The disclosure of EPO942,405 A2 is directed at isolating the islands to provide a buffermechanism to maintain an electric field for an extended period of time,theoretically forever. The object of the present invention is not tocompletely isolate each electrode 52 in the array 40 to retain charge.Rather, the present invention allows the tuning of the degree ofelectrical isolation of the electrodes. Tuning the degree of isolationof each electrode is accomplished by controlling the sidewall angles,sidewall slope, mesa height, mesa shape; valley width, valley depth andsputtering conditions. The specific geometry of the mesa shaped segments44 will determine the amount or thickness of conductive coating on thearray layer 38, thereby affecting the amount of resistance betweenelectrodes 52.

The electrode film 12 of the present invention has been described foruse with gyricon media. However, workers skilled in the art willappreciate that the electrode film 12 is equally applicable for use inconjunction with other types of electric field addressable media, suchas particulate electrophoretic displays, which are available from E-InkCorporation.

Thus far the electrode film of the present invention has only beendiscussed as a permanent part of the display media. In another optionalembodiment, the electrode film could be separate from the Gyricon orparticulate electrophoretic displays, such as available from E-InkCorporation. The electrode film 12 of the present invention can beformed entirely separate from the display, as can the backside electrode16. When a user desires to write or mark on the display, the displaywould be positioned by the user between the backside electrode 16 andthe electrode film 12. After writing, the user would separate thedisplay from the backside electrode 16 and electrode film 12. The imagewritten on the display would then be substantially indelible until theuser decided to rewrite by again positioning the display between thebackside electrode 16 and the electrode film 12.

The electrode film 12 of the present invention can also be used in aprinter for printing on an electrode-less medium. As one example shownin FIG. 7, an electrode-less gyricon medium 18 could be fed between abackside electrode roller 60 and an electrode film roller 62. Theelectrode film roller 62 could use the electrode film 12 as its outersurface. An activation mechanism (not shown) would impart charge to theselected electrodes 52 immediately prior to the nip between the backsideelectrode roller and the electrode film roller, with the activatedelectrodes generating the electric field which “writes” on theelectrode-less gyricon medium at the nip.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. An electronic display comprising: a display panel having a viewing surface and a backside surface opposing the viewing surface, the display panel comprising: particles which are optically anisotropic toward the viewing surface and responsive to changes in magnetic field; and a surrounding media containing the particles; an array layer in contact with the viewing surface of the display panel, the array layer being formed of an electrically non conductive material, wherein the array layer has a plurality of mesa shaped segments, each mesa shaped segment comprising: a top face; and side walls extending downward from the top face, wherein each of the mesa shaped segments provide a support structure for one of the electrodes; and an electrode layer coating the array layer, the electrode layer formed of an electrically conductive material, the electrode layer exposed for contact by an electrical stimulus, the electrode layer providing a plurality of electrodes, each electrode of the electrode layer being partially electrically isolated from the other electrodes in the electrode layer, wherein the electrode layer is thicker on the top face than on the side walls producing resistive bridges between adjacent top faces, the resistive bridges partially electrically isolating each electrode from the other electrodes in the electrode layer.
 2. The electronic display of claim 1, further comprising a stylus electrically connected to address select electrodes of the electrode layer.
 3. The electronic display of claim 1, wherein the electrode layer is ITO.
 4. An electronic display comprising: a display panel having a viewing surface and a backside surface opposing the viewing surface, the display panel comprising: particles which are optically anisotropic toward the viewing surface and responsive to changes in magnetic field; and a surrounding media containing the particles; an array layer in contact with the viewing surface of the display panel, the array layer being formed of an electrically non conductive material; an electrode layer coating the array layer, the electrode layer formed of an electrically conductive material, the electrode layer exposed for contact by an electrical stimulus, the electrode layer providing a plurality of electrodes, each electrode of the electrode layer being partially electrically isolated from the other electrodes in the electrode layer; and an electrode in contact with the backside surface of the display panel; wherein the array layer provides an array of mesa shaped segments, wherein each mesa shaped segments provide a support structure for one of the electrodes, each mesa having a substantially flat top face and four side walls extending downward defining valleys relative to the top face, the sidewalls recess at included angles of 70 degrees or greater relative to the top faces; and wherein the electrode layer has an electrode thickness over the mesa shaped segments of sufficient thickness and sufficient conductivity to form substantially conductive electrode plates, the electrode layer having a top face thickness and a valley thickness over the sidewalls, the sidewall thickness being thinner than the top thickness and of sufficient thinness and sufficient resistivity to form resistive bridges between adjacent electrode plates.
 5. The electronic display of claim 4, further comprising a stylus electrically connected to address select electrodes of the electrode layer.
 6. The electronic display of claim 4, wherein the electrode layer is ITO (Indium Tin Oxide).
 7. A electrode film comprising: a sheet formed of a substantially electrically insulative material, the sheet having a textured surface, the textured surface providing a plurality of mesa structures each having an electrode face extending generally in the plane of the textured surface and peripheral side walls around the electrode face defining valleys relative to the electrode face; and a conductive layer deposited over the textured surface of the sheet, the conductive layer having an electrode thickness over the electrode faces of the mesa structures of sufficient thickness and sufficient conductivity to form substantially conductive electrode plates, the conductive layer having a valley thickness over the peripheral side walls which is thinner than the electrode thickness and of sufficient thinness and sufficient resistivity to form resistive bridges between adjacent electrode plates.
 8. The electrode film of claim 7, wherein the electrode film is disposed in a printer.
 9. The electrode film of claim 7, wherein the sheet and conductive layer are substantially transparent or translucent to visible light.
 10. The electrode film of claim 7, wherein the sidewalls recess at included angles of 70 degrees or greater relative to the electrode faces.
 11. The electrode film of claim 7, wherein substantially all mesa structures are coated with the conductive layer by a sputtering process.
 12. The electrode film of claim 11, wherein the conductive layer is ITO (Indium Tin Oxide).
 13. The electrode film of claim 12, wherein the ITO conductive layer is up to about 4 nm thick.
 14. A method of making an electrode film substrate, the method comprising: texturing a surface of a substrate into an array of mesa shaped segments having a repeatable mesa size and repeatable distance between each mesa shaped segment, the substrate being electrically non conductive; and depositing a conductive coating on the surface of the substrate thereby forming an electrode layer with one electrode on each mesa shaped segment and resistive bridges surrounding each electrode to at least partially electrically isolate each electrode from the other electrodes in the electrode layer.
 15. The method of claim 14, wherein the mesa shaped segments have a substantially flat top face and side walls extending downward from the top face, wherein the sidewalls recess at included angles of 70 degrees or greater relative to the top faces, each mesa side wall intersects with an adjoining mesa side wall to form a valley relative to the top face.
 16. The method of claim 15, wherein the depositing step is performed by a sputtering process resulting in the top face electrode layer to be thicker than the side wall electrode layer to thereby forming resistive bridges between adjacent top faces, the resistive bridges surrounding each top face to at least partially electrically isolate each top face from the other top faces in the array.
 17. The method of claim 16, wherein the conductive coating is ITO (Indium Tin Oxide). 